1. Field of the Invention
The present invention generally relates to a rolling bearing assembly for rotatably supporting a vehicle wheel and, more particularly, to the wheel support bearing assembly incorporating a rotation sensor mounted on a sensor cap closing one open end of the bearing, for detecting the number of revolutions of the vehicle wheel.
2. Description of the Prior Art
Some of the vehicle wheel bearing assemblies are known, in which a rotation detecting device for detecting the number of revolutions of vehicle wheels is incorporated for the purpose of performing various controls, for example, for controlling an automobile anti-skid control system, also known as an anti-lock brake system (ABS), and a traction control system (TCS). This rotation detecting device includes a pulsar ring fitted to a rotatable member of the bearing assembly for rotation together therewith and a sensor fitted to a stationary member of the same bearing assembly. In the case of a vehicle driven wheel having the sensor incorporated on an inboard side, the sensor is generally installed in a sensor cap used to close one of opposite open ends of an annular bearing space delimited between the rotatable and stationary members. See, for example, the Japanese Laid-open Utility Model Publication No. 7-31539, the Japanese Laid-open Patent Publication No. 9-61443 and the Japanese Patent No. 3440800.
The wheel support bearing assembly, in which the sensor is installed in the sensor cap, is illustrated in FIGS. 3 and 4. The wheel support bearing assembly shown therein is of a structure designed to rotatably support a vehicle driven wheel and includes an outer member 31 rigid with a vehicle body structure, and an inner member 32 having a vehicle wheel secured thereto for rotation together therewith, and a plurality of, for example, two, rows of rolling elements 3 accommodated within an annular bearing space delimited between the inner and outer members 31 and 32 and spaced a distance axially from each other. The inner member 32 is made up of an hub axle and an inner race forming segment 35 fixedly mounted on an inboard end of the hub axle as shown in FIG. 3. A pulsar ring 39 is secured to the inner race forming segment 35 and, on the other hand, a sensor cap 43 made of a synthetic resin is secured to the outer member 31 so as to close an inboard open end of the annular bearing space between the outer and inner members 31 and 32.
The sensor cap 43 is provided with the sensor 40 therewith and arranged so as to confront the pulsar ring 39 in a direction axially of the bearing assembly, and the sensor 40 cooperates with the pulsar ring 39 to define a rotation sensor unit 38. A portion of FIG. 3 indicated by the phantom circle IV is shown on an enlarged scale in FIG. 4. As shown therein, the sensor 40 is fitted to a sensor support member 49 and is fixedly inserted into a sensor insertion hole 46 defined in the sensor cap 43 with the sensor support member 49 arranged on an outer surface of the sensor cap 43. When the sensor support member 49 is firmly fastened to the sensor cap 43 by means of at least one sensor fastening bolt 52, the sensor 40 can be firmly locked to the sensor cap 43. This fastening of the sensor support member 49 with the sensor fastening bolt 52 is carried out by threading the sensor fastening bolt 52 into an insert nut 45 embedded in the sensor cap 43.
It has, however, been found that the foregoing construction has the following problem associated with an undesirable ingress of foreign matter into the bearing assembly. Specifically, foreign matter such as, for example, muddy water may ingress through slight gaps between the internal threads of the insert nut 45 and the sensor fastening bolt 52 and surfaces of contact between the sensor fastening bolt 52, the sensor support member 49 and the sensor cap 43 and then into the interior of the bearing assembly, and particularly the annular bearing space, through an axial bottom hole 42 defined in the sensor cap 43. The foreign matter such as, for example, muddy water may also ingress through surfaces of contact between the insert nut 45 and a portion of the sensor cap 43 made of the synthetic resin. Once such ingress of the foreign matter into the interior of the bearing assembly occurs, not only will the lifetime of the bearing assembly be lowered, but also the pulsar ring may possibly be deteriorated.